Described herein are methods for sintering metal components, particularly steel components, using a controlled atmosphere. More particularly, described herein are methods for sintering steel components using an atmosphere comprising nitrogen and hydrogen and a method for pre-conditioning a metal belt prior to its operation in a sintering furnace.
Powder metallurgy is routinely used to produce a variety of simple- and complex-geometry carbon steel components requiring close dimensional tolerances, good strength and wear resistant properties. The technique involves pressing metal powders that have been premixed with organic lubricants into useful shapes and then sintering them at high temperatures in continuous furnaces into finished products in the presence of controlled atmospheres.
The overall cost of producing components by powder metallurgy has been known to be greatly affected by both the time and money spent on maintaining furnaces and by the cost of controlled atmospheres. The productivity and quality of components, on the other hand, are affected by furnace downtime and consistent composition of controlled atmospheres, respectively. Therefore, there is a need to develop methods and/or atmospheres that will assist in reducing downtime and maintenance costs and improving quality and productivity of components produced by powder metallurgy.
The continuous sintering furnaces normally contain three distinct zones, i.e., a preheating zone, a high heating zone, and a cooling zone. The preheating zone is used to preheat components to a predetermined temperature and to thermally assist in removing organic lubricants from components. The high heating zone is used to sinter components, and the cooling zone is used to cool components prior to discharging them from continuous furnaces.
The high heating zones of continuous furnaces used for sintering steel components are generally operated at temperatures above about 1,832° F. (about 1,000° C.). Because of high temperature operation, expensive, high temperature nickel-chromium containing alloys such as Inconel are used for building high heating zones of continuous furnaces. The use of these expensive, high temperature alloys helps in prolonging life of continuous furnaces and concomitantly reducing maintenance costs. Alternatively, relatively inexpensive stainless steels can also be used to build sintering furnaces. However, the later stainless steels have a shorter operative life than the high temperature nickel-chromium alloys.
The continuous mesh belts used to load and unload components in continuous furnaces are generally made of either expensive, high temperature nickel-chromium containing alloys such as Inconel or relatively inexpensive stainless steels. The expensive, high temperature nickel-chromium containing alloys are preferred materials for building wire mesh belts and obtaining longer life, but they are cost prohibitive and seldom used by the Powder Metal Industry. Stainless steel wire mesh belts are usually selected for sintering of steel components because of their high temperature properties and lower cost than the expensive alternatives, such as high temperature nickel-chromium containing alloys. Although stainless steel mesh belts require frequent maintenance, they are commonly used by the Powder Metal Industry because they are relatively inexpensive.
The controlled atmospheres used for sintering steel components are generally produced and supplied by endothermic generators, ammonia dissociators, or blending pure nitrogen with hydrogen. The endothermic (“endo-gas”) atmospheres are produced by catalytically combusting controlled amount of a hydrocarbon gas, such as natural gas in air in endothermic generators. The endothermic atmospheres typically contain nitrogen (about 40%), hydrogen (about 40%), carbon monoxide (about 20%), and low levels of impurities, such as carbon dioxide, oxygen, methane, and moisture. The atmospheres produced by dissociating ammonia contain hydrogen (about 75%), nitrogen (about 25%), and impurities in the form of undissociated ammonia, oxygen, and moisture.
Nitrogen-hydrogen atmospheres produced by blending pure nitrogen with hydrogen have been used by the Powder Metal Industry for more than 30 years as alternatives to endothermically generated and dissociated ammonia atmospheres. Because these atmospheres are produced by blending pure nitrogen and hydrogen, they avoid problems associated with the exposure of workers to environmentally unfriendly and harmful gases. Furthermore, since the composition and flow rates of these atmospheres can be easily changed and precisely controlled, they have been widely accepted by the Powder Metal Industry for sintering steel components that require good carbon control, consistent quality and properties. U.S. Pat. No. 5,613,185, for example, disclosed nitrogen-hydrogen based atmospheres that include the use of a controlled amount of an oxidizing agent such as moisture, carbon dioxide, nitrous oxide, or mixtures thereof along with nitrogen-hydrogen containing atmospheres.
The nitrogen-hydrogen atmospheres are reducing to the sintered steel and to the stainless steel of the belt. Although pure nitrogen-hydrogen atmospheres containing less than 5 parts per million (ppm) oxygen and −80° F. (−62° C.) dew point (less than 10 parts per million (ppm) moisture) have been very useful in producing steel components with good quality, consistency, and properties, they have been found to impact negatively on the life of wire mesh belts made of both expensive, nickel-chromium containing alloys and relatively inexpensive stainless steels, thereby increasing downtime and maintenance costs. Therefore, there is a need to develop improved nitrogen-hydrogen based atmospheres for producing steel components by powder metallurgy with consistent quality and properties while improving life of wire mesh belts and reducing downtime and maintenance costs.